Acid-fixing bath



Patented Jan. 6, 1925,

ATENT osrics.

SAMUEL E. SHEPPARD, OF ROCHESTER, NEW YORK, ASSIGNOR TO EASTMAN KODAKCOMPANY, OF ROCHESTER, NEW YORK, A CORPORATION OF NEW YORK.

AoIiJ-FIXING BATH.

No Drawing. Application filed May 6,

To all whom it may concern:

Be it known that I, SAMUEL E. SHEPPARD, a subject of the King of GreatBritain, residing at Rochester, in the county of M911- roe and State ofNew York, have invented certain new and useful Improvements inAcid-Fixing Baths, of which the following is a full, clear, and exactspecification.

This invention relates to photographic acid fixing baths. One object istoinclude in such a bath only materials which will not inhibitthe actionof any hardener that may be present, nor form objectionable com-. poundsor 'precipitates'with any of the materials present. Another object ofthis in vention is to provide such baths made from constituents whichmay be prepared in the form of a single dry powder, which will notdeteriorate under ordinary conditions or within areasonable time.Another object is to provide such a powder which will not contain'anacid, as such, but which will when dissolved produce an acid through thedecomposition of the substances. Another object is to provide in such abath a buffer material, of the nature of'those described and claimed inthe patent of Felix A. Elliott, No. 1,411,687, granted Apr. 4, 1922, bywhich the acidity of the bath is main tained within desired limits. Asthere pointed out, the fixing bath should have an acidity or hydrogenion concentration not greater than pH=4 and not less than pH:6.5 or,preferably, pI-I:6.

These and other objects, and the manner in which they are attained, willbe more fully set forth in the following specification and claims. Inorder to make the invention clear, I will first mention a few well knowncharacteristics of such baths and then point out the principles ofmyinvention.

The basis of practically all fixing baths is sodium thiosulfate or hypo,the best known solvent .for photographic silver salts. In operation,there is brought over from the developing bath a considerable amountofalkaline developer, 'and as a preservativeto prevent the harmfuloxidation of the de veloper, a sulfite, usually sodium sulfite, is

customarily added to the fixing bath. To the reversible equationsconnecting these 100 1921. Serial No. 467,308.

increase the preservative action and also to neutralize the alkali, anacid is also usually added. Hypo has a tendency in an acid solu tion todecompose into sulfurous acid and sulfur, but this is prevented if there,is enough sulfite present to maintain the bath in equilibrium.Sometimes an acid bisulfite, such as sodium bisulfite, N aHsO orpotassium metabisulfite,;K S,O is used as a preservative in a fixingbath. .-It is also organic acids, which are solid at all ordinarytemperatures It has not been possible to furnish an acetic acid bath inpowder form because of the low melting point of this acid. It has beenunsatisfactory, moreover, to mix in a single powder all the ingredientsincluding hypo and an acid, because when these in finely divided formare intimately mixed the hypo is decomposed. It has been customarytherefore, to furnish two separate powders.

I have discovered that the acid asa separate ingredient may be omittedentirely from such'a, mixed powder, by the substitution, in place ofsulfite and an acid, of a bisulfite and the salt of an organic acid,which when dissolved produce a bath containing bisulfite,sulfite,sulfurous acid, an organic acid and a buffer salt of the organicacid. If, for instance, sodium bisulfite and sodium acetate aredissolved, the resulting bath contains sodium bisulfite, sodium sulfite,sulfurous acid, sodium acetate, and acetic acid.- For purposes ofillustration and without committing myself to an exact statement ofreactions that would occuris a fixing bath,

v substances only may be assumed to be as follows:

' within certain limits as described in the above identified Elliottpatent.

This action may, of course, be attained by the use of many other saltssuch as fol-mates, citrates, tartrates, etc.

The dissociation necessary is attained only when the salts are in truesolution as distinguished from suspension or so-called colloidalsolution, and obviously, only such salts are suitable as enter into atrue'solution in water. 7

When the bisulfite and-salt are dissolved together with hypo, thereaction is, of course, more complicated, butthe nature of the reactionis apparent from the above explanatory remarks.

The reaction is still more complicated when a hardener is alsointroduced. It has long been known that alum has a hardening effect andthat the alum bath should be acid. It has been generally assumed thatmany acids, particularly organic acids, could be a used with equallysatisfactory results; but

I have discovered that the presence in appreciable quantities ofpolybasic and hydroxy-organic acids, such as citric, tartaric, lact1c,ma-lic, maleic, salicylic, etc., acids or their salts inhibits thehardening action of alum upon the gelatine of photographic emulsions.This is due to the fact that theseaacids, or rather their anions, formstable complexes with aluminium, chromium and iron (the latter beingfrequently present as an impurity in commercial alums) whereby thecombination of the hydrous oxides with the gelatlne 1s prevented. The"monobasic fatty acids and their alkali metal salts and partlcularlyformic and acetic acids and their alkali metal salts, do not formcomplexes w1th aluminium of sufficient stability to inhibit hardening,but they do stabilize sufficlently to prevent the precipitation ofaluminium hydroxide. For this reason, where, as is usually the case, analum is present, a formate or acetate is very much preferable tosalts-of acids of the classes specified above. When I refer to thealkali metalsI intend to include ammonium.

I have further discovered that sodium bisulfite or commercial potassiummetabisulfite, which hitherto could notbe used satisfactorily with analum containing an aluminium compound because of the formatlon ofinsoluble basic aluminium c mpounds, can'be used with such alums if anacetate or formate is also present. A typical formula is the following,the dry powdered salts being mixed in the order given,-

. Grams. Sodium thiosulfate 240 Sodium acetate 55 Sodium bisulfite 30Potassium alum 8 of the bisulfite and the acetate or other salt; so thatthese constitute substantially the only source of acid.

Where I have referred to the materials as in the form of a powder, Idonot mean to limit myself to any particular degree of fineness of theparticles and rather coarse crystals or lumps of material may be used.These if desired may be mixed in an inert binding material such as gumarabic and formed into tablets or a paste.

The use of such gums appear to be especially desirable where commercialsalts are used, containing impurities. The gum tends to keep anyprecipitates in a harmless suspended form. At the same time smallamounts of alkali citrates or tartrates, or/a-nd salts of thecorresponding acids may be included replacing acetate and bisulfite toimprove pro rata the clarity of the bath, but the quantity of these mustbe small if the hardening action of the alum is not to be greatlyreduced.

It is apparent that numerous modifications and equivalents are possible,and I contemplate these as within the scope of my invention as definedin the appended claims.

Having thus described my invention,

what I claim. as new and desire to secure by Letters Patent, is: 1.Powdered material adapted'when dissolved to constitute a photographicfixing bath and comprising sodium thiosulfate, a bisulfite of an alkalimetal, and a alkali metal salt of a carboxylic acid, said salt havingthe property of entering into a true solution in water.

2. Powdered material adapted when dissolved to constitute a photographicfixing bath and comprising sodium thiosulfate, a

bisulfite of an alkali metal, and an alkali metal salt of a carboxylicacid, said salt having the properties of entering into a true solutionin water and, when in true aqueous solution in equilibrium with thebisulfite of acidfying the bath and of maintaining the hydrogen ionconcentration of the bath at a predetermined strength greater thanpH:6.5.

3. Powdered material adapted when dissolved to constitute a photographicfixing bath and comprising sodium thiosulfate, a bisulfite of -an alkalimetal and an alkali metal salt of a carboxylic acid having not over ninecarbon atoms.

4:. Powdered material adapted when dissolved to constitute aphotographic fixing bath and comprising sodium thiosulfate, a bisulfiteof an alkali metal and an ,alkali metal salt of a monobasic carboxylicacid having not over nine carbon atoms.

5. Powdered material adapted when dissolved to constitute a photographicfixing bath and'comprising sodium thiosulfate, a bisulfite of an alkalimetal and an alkali metal salt of a monobasic carboxylic acid, said salthaving the property of entering into a true solution with water.

6. Powdered material adapted when dissolved to constitute a photographicfixing bath and comprising sodium thiosulfate, a bisulfite of an alkalimetal and an alkali metal salt of a mon'obasic carboxylic acid, saidsalt having the properties of entering into a true-solution with waterand, when in true aqueous solution in equilibrium with the bisulfite, ofacidifying the bath and of maintalnlng the hydrogen lon concentration ofthe bath at a predetermined strength greater than pH:6.5.

7. Powdered material adapted when dissolved to constitute a photographicfixing bath and comprising sodium thiosulfate, a bisulfite of an alkalimetal and an alkali metal salt of a lower fatty acid.

8. Powdered material adapted when-dissolved to constitute a photographicfixing bath and comprising sodium thiosulfate, a bisulfite of an alkalimetal and an, alkali metal salt of an acid of the aliphatic serieshaving not over two carbon atoms.

9. Material in the form of a single powder adapted when dissolved toconstitute a photographic fixing bath and comprising sodium thiosulfate,a bisulfite, of an alkali metal and an acetate of an alkali metal.

10. Powdered material adapted when dissolved to constitute aphotographic fixing bath and comprising sodium thiosulfate, a hardenerand an alkali metal salt of a carboxylic acid, said salt having theproperty of entering into 'a true solution in water.

11. Powdered material adapted when dissolved to constitute aphotographic fixing bath and comprising sodium thiosulfate, a hardenerand an alkali metal salt of a monobasic carboxylic acid, said salthaving the property of entering into a true solution with water.

12. Powdered material adapted when dissolved to constitute aphotographic fixing bath and comprising sodium thiosulfate, a hardenerand an alkali metal salt of a monobasic carboxylic acid, said salthaving the properties of entering into a true solution with water and,when in true aqueous solution in equilibrium with the bisulfite, ofacidifying the bath and of maintaining the hydrogen ion concentration ofthe bath at v a predetermined strength greater than pH=6. t

13. Powdered material in a single mixture adapted to constitute whendissolved a photographic acid fixingbath and comprising sodiumthiosulfate, a hardener, a bisulfite of an alkali metal, and an alkalimetal salt of a lower fatty acid.

14. Material'in the form of a single powder adapted to constitute whendissolved a photographic acid fixing bath and comprising sodiumthiosulfate, a hardener, a bisulfite of an alkali metal, and an alkalimetal salt of a fatty acid containing not over two carbon atoms.

15. Powdered material in the form of a single mixture adapted toconstitute when. dissolved a photographic. acid fixing bath andcomprising sodium thiosulfate, a hard ener, a bisulfite of an alkalimetal and an acetate of an alkali metal.

16. Powdered material in the form of a single mixture adapted whendissolved to constitute a photographic acid fixing bath, and comprisingsodium thiosulfate, an alum, a bisulfite of an alkalimetal, and analkali salt of a lower fatty acid.

17. Material in the form of a single powder adapted when dissolved toconstitute a photographic acid fixing bath, and comprising sodiumthiosulfate, an alum containing an aluminium compound. a bisulfite of analkali metal, and an alkali metal salt of a fatty acid containing notover two carbon atoms.

18. Material in the form of a single powder adapted when dissolved toconstitute a photographic acid fixing bath, and comprising sodiumthiosulfate, an alum, a bisulfite of an alkali metal, and an acetate ofan alkali metal, said acetate having the property of entering into atrue solution in water. i

19. Material in the form of a single powder adapted in aqueous solutionto constitute a photographic acid fixing bath and comprising sodiumthiosulfate, an alum, a bisulfite of an alkali metal and an acetate ofan alkali metal, the powder being substantially free from free acid.

20. Material in the form of a single powder adapted when dissolved toconstitute a photographic acid fixing bath, and comprising sodiumthiosulfate, alum, a bisulfite of an alkali metal and an acetate of analkali metal.

21. Material in the form of a single powder adapted when dissolved toconstitute a,

photographic acid fixing bath and consisting of sodium thiosulfate, ahardener, a bisulfite of an alkali metal, and an acetate of an alkalimetal. 1

Signed at Rochester, New York, this 3rd day of May, 1921.

SAMUEL E. SHEPPARD.

